Butterfly valves, such as are used, for example, in the low-pressure region of steam turbines, are actuated by a servo-motor, with opening taking place by means of hydraulic force and closing by means of spring force. An open central butterfly valve with symmetrical wings is free from torque. When the butterfly wings are in an oblique position, the flowing medium exerts a torque which always tends to close the butterfly. The closing spring has the task of setting the butterfly into motion from the neutral open position, i.e., of accelerating the mass of the butterfly wings and all the moving parts. If a short closing time is required, and if the mass moment of inertia of the butterfly system is large, the acceleration time can account for a substantial part of the total closing time. The hydraulic force must overcome the torque due to the flow and the force of the closing spring. If a stronger spring is selected in order to decrease the closing time, the hydraulic force and hence the servo-motor must be larger.